1. Technical Field of the Invention
The present invention relates to testing equipment for telecommunications equipment. More particularly, and not by way of any limitation, the present invention is directed to an AC power fault machine capable of testing telecom line cards and broadband coaxial cable interfaces to known power fault immunity criteria.
2. Description of Related Art
Telecommunications (telecom) equipment deployed in today""s networks is required to comply with various governmental and industry standards not only to ensure seamless interoperability which reduces the risk of service interruption resulting from third-party product failures, but also to address various product safety issues. Accordingly, equipment manufacturers test their products to telecom industry standards commonly known as BellCore specifications (also sometimes referred to as Telcordia specifications) which define an extensive list of electromagnetic compliance (EMC), product safety, and environmental requirements.
The BellCore specifications comprise two sets of testing standards, GR-1089-CORE and GR-63-CORE. The tests in GR-1089-CORE deal primarily with electrical phenomena, whereas the tests in GR-63-CORE are predominantly environmental in nature. While each set of standards is quite extensive, typically only a subset of the tests are required based on the type of equipment and its intended operating environment. Together, these two sets of standards specify the electrical and environmental requirements that networking hardware must meet in order to be located in a telco building, e.g., the telco""s central office (CO).
Besides the testing requirements, which are determined by product type, BellCore has defined additional testing levels generally referred to in the telecom industry as Telcordia""s Network Equipment Building Systems (NEBS) levels. The appropriate NEBS level for a particular equipment is determined, again, by its intended operating environment and specific requirements of the Regional Bell Operating Companies (RBOCs). Generally, a higher NEBS level indicates a more stringent testing specification.
NEBS testing verifies that telecom equipment can operate successfully under certain electrical and physical environmental stresses and not pose a safety hazard to personnel and users. These stresses and hazards include earthquakes, airborne contaminants, fire and smoke, electromagnetic interference (EMI), electrical safety, and grounding.
Requirements under the three NEBS levels may be summarized as follows: Level 1 includes: electrical safety; lighting and AC power fault (2nd level); bonding and grounding; emissions; and fire resistance; Level 2 includes: all of Level 1 in addition toxe2x80x94electrostatic discharge (ESD) under normal operation; emissions and immunity; lighting and AC power fault (1st level); ambient temperature and humidity (operating); earthquake Zone 2 and office vibration; and airborne contaminants (indoor level); Level 3 includes: all of Level 1 and Level 2 in addition toxe2x80x94ESD (installation and repair); open door emissions and immunity; ambient temperature and humidity (short-term); earthquake Zone 4; airborne contaminants (outdoor level); and transportation and handling. Each test within these three Levels is defined in either the GR-1089-CORE or GR-63-CORE documentation.
Testing of telecom interfaces, i.e., tip-and-ring (T and R) interfaces of the line cards utilized in telecom equipment and broadband coaxial cable interfaces, for lightning and AC power fault immunity in accordance with the above-referenced standards is necessary for several reasons. Power companies, Local Exchange Carriers (LECs) and broadband access providers often serve the same customers, and frequently employ joint-use facilities such as supporting structures or a common trench for their respective outside plant. Metallic conductors, such as cable or wire pairs serving telecom equipment may be exposed to electrical surges resulting from lightning and commercial power system disturbances. Despite the presence of protective devices in the telecommunications network that limit the effect of lightning and power surges, a portion of these disturbances can be impressed on the network equipment. Accordingly, under abnormal conditions, for instance, the power and telecommunications lines (including coax cables) may come into electrical contact. If the contact occurs to a primary power line, faults may be cleared quickly by the power system (5 seconds or less), and protectors (e.g., carbon blocks) can limit 60 Hz voltages appearing on the T or R conductors to maximum of approximately 600 VRMS with respect to ground. If the contact occurs to a secondary power line, the full secondary voltage with respect to ground (up to about 275 VRMS in some cases) may appear on the T and R conductors, which may persist indefinitely as the secondary fault may not be cleared by the power system.
Moreover, because electric power lines and telecom lines often occupy parallel routes as a result of a common right-of-way, the magnetic field produced by currents in a nearby power line, especially under abnormal conditions such as a phase-to-ground fault, may result in large voltages being induced into the telecom lines through electromagnetic coupling. The induced voltages appear longitudinally in the T and R conductors and may approach several hundred volts. Lower levels of induction may result from a high-impedance power fault such as a phase conductor falling to the earth. If the resulting unbalanced current is within the normal operating range of the power system, or if power system breakers or fuses do not operate, the fault may persist for an extended period of time.
Under the BellCore""s GR-1089-CORE standard, the lightning surge and AC power fault immunity criteria include compliance with various tests such as short-circuit tests (tip to ring, tip to ground with ring open-circuited, ring to ground with tip open-circuited, tip and ring to ground simultaneously, et cetera) and several AC power fault tests. As set forth hereinabove, these criteria are separated into 1st level and 2nd level criteria. To comply with the 1st level criteria, it is required that the telecom equipment under test (i.e., EUT) be undamaged and continue to operate properly after power stress is removed. To comply with the 2nd level criteria, primary protectors are typically removed and high open-circuit voltages and high short-circuit currents are often applied for variable durations, ranging up to 15 to 30 minutes or so in some instances. The EUT may sustain damage, but it is required that the equipment not become a fire, fragmentation (that is, forceful ejection of fragments), or an electrical safety hazard.
While several lightning machines are available for conducting the lightning compliance tests required under the BellCore standards alluded to hereinabove, there is a paucity of appropriate AC power fault (PF) machines capable of sourcing power to telecom units under test for adequately conducting the AC power fault compliance tests, including the 2nd Level tests. Further, the relatively few solutions extant today are beset with various shortcomings and drawbacks. First, the existing AC power fault machines are typically custom-designed to a large extent and, accordingly, incapable of accommodating various telecom equipment types and form factors. Additionally, these machines are quite expensive to manufacture owing at least in part to their custom design. In spite of the custom design, however, the existing PF machines are not capable of providing appropriate levels of test power safely to the EUT to conduct the whole range of 2nd Level power failure tests as required under the relevant Sections of the GR-1089-CORE standard. Furthermore, although the conventional PF machines are fairly capacious because of the large size of the transformers typically required to provide adequate levels of test power, they are incapable of sourcing power to both two-wire T/R interfaces as well as broadband coax cable interfaces in the same physical plant.
Accordingly, the present invention advantageously provides a safe, versatile and single-platform power fault (PF) testing apparatus that is capable of testing both line cards"" T/R interfaces (two-wire interfaces) as well as broadband coaxial cable interfaces to BellCore""s 2nd Level AC power fault standards. The apparatus is powered by a three-phase 480 VAC, 600 A shore power service. A plurality of transformers and load resistor banks are coupled to the power source in a network configuration that is organized as a plurality of selectable and switchable power paths for interfacing to T/R or broadband coaxial cable interfaces of the EUT disposed in a test chamber. Power relays are provided for interconnecting the transformers and load resistor banks in the network arrangement. Computer-controlled output relays are included for effectuating precise and repetitive time duration control of output power applied to the telecom interfaces under test. A remote control station is provided whereby test personnel may monitor and control all parameters of the PF apparatus.
In the presently preferred exemplary embodiment of the present invention, a motor-driven variable autotransformer unit is coupled to the shore power source through a power relay. The variable autotransformer unit is operable to provide a power output that is selectably switchable to a first load resistor bank by means of one or more power relays. A two-wire T/R interface of the EUT disposed is controllably powered by means of an output relay coupled to the first load resistor bank that is preferably provided as a collection of binary-coded resistance values in one or more branches. The branch resistance is variably selectable by remotely controlled relays.
A first fixed transformer is selectably coupled to the output of the variable autotransformer unit in a current boost configuration. The first fixed transformer is operable to provide selectably switchable power to a second load resistor bank, wherein the second load resistor bank interfaces to a coaxial cable interface via an output relay. One or more power relays interconnect the first fixed transformer to the second load resistor bank which is preferably operable to effectuate a network of multiple current paths, each having a predetermined current step. A set of relays, remotely controllable by means of switches, control the selection of the applied current path to the coaxial cable interface.
A second fixed transformer is selectably coupled to the output of the variable autotransformer unit via an output relay in a voltage bucking configuration, whereby the second fixed transformer is operable to interface directly with the coaxial cable interface to apply appropriate levels of current for conducting certain additional tests under the GR-1089-CORE standard.
As a further aspect of the present invention, a third fixed transformer is disposed on a separate branch between the first fixed transformer and second load resistor bank via at least one power relay. Accordingly, the third fixed transformer is operable to provide selectably switchable power at a different setting to the second load resistor bank via the separate branch under computer-based timer relay control.
In a still further aspect, the PF apparatus of the present invention includes a resistive interface circuit with a grounded return for interfacing with the line card""s T/R interface for conducting certain 1st Level AC power fault tests (xe2x80x9cObjective Testsxe2x80x9d) in addition to the 2nd Level tests. The resistive interface circuit is controlled through the output relay coupled to the first load resistor bank. A 1:2 step-up transformer with a ground on its secondary coil side is coupled to the output of the output relay for appropriately powering the grounded resistive network.